Theory:

Pure substances are those that are completely made up of one form of particle. Physical processes cannot distinguish pure substances from other forms of matter.
Example:
Salt, sugar, oxygen, copper, iron etc.,
Based on the kind of atom, we classify pure substances as
• Elements and
• Compounds
Compound is a form of matter created by combining two or more elements in a specific mass ratio. To decompose a compound into its constituent components chemical methods are used.
Water$$H_2O$$, Oxygen $$O_2$$, Nitrogen dioxide $$NO_2$$, Salt $$NaCl$$, and so on.
Elements are the fundamental substances that cannot be separated into simpler substances by any chemical methods. All the elements in the periodic table comes under this category such as helium, hydrogen, oxygen, iron, etc.
In $$1661$$, Robert Boyle became the first scientist to use the term element.

Robert Boyle

A French chemist named Antoine Laurent Lavoisier $$1743$$ - $$1794$$ was the first to define an experimentally useful description of an element.

Antoine Laurent Lavoisier
An element, according to him, is a fundamental type of matter that cannot be broken down into simpler substances by chemical reactions.
Based on the physical and chemical properties, elements are classified as
• Metals
• Non-metals
• Metalloids
Let us now identify some of the elements and compound of a pure substance:

 Pure Substance Element /Compound Reason why it is compound or element Water $$H_2O$$ Compound Can be separated asWater = Hydrogen + Oxygen Lead $$Pb$$ Element Cannot be separated Phosphorous $$P_4$$ Element Cannot be separated Sodium Chloride $$NaCl$$ Compound Can be separated asSodium Chloride = Sodium + Chloride Sulphur $$S_8$$ Element Cannot be separated